This invention relates to tape motion control apparatus and more particularly to such apparatus for use in moving magnetic tape at high acceleration rates without slippage from one reel to another, and thereafter at constant velocity and with constant tension in an unbuffered reel-to-reel tape transport system.
The referenced related patent application discloses a tape drive apparatus employing a finely graduated tachometer on an idler roll in the tape feed path to measure the amount of tape advanced during a complete revolution of each tape reel shaft in an unbuffered reel-to-reel tape drive system. From the amount of tape advanced between single pulses per revolution of each reel shaft, the radius of such reel was independently derived and continually modified as radius changed during feeding of the tape. From a table storing inertial values corresponding to predetermined values of tape reel radius, the radius values for each reel were repeatedly converted into corresponding inertial values by table lookup. From these inertial values, drive currents for each reel motor were calculated so as to provide a given start and stop tape motion and maintain velocity and tape tension constant during the read/write process and maintain tape from drifting during a stop-lock mode. This arrangement operates very satisfactorily at tape acceleration rates up to 400 inches per second. However, when the acceleration rates are increased tenfold, slippage of the tape can occur at the tachometer in the tape feed path, with resultant loss of precise control of tape tension and velocity.
U.S. Pat. No. 3,764,087 and U.S. Pat. No. 3,984,868 disclose reel-to-reel tape motion control systems which employ separate drive motors for each reel and require no tape buffering or tachometer in the tape feed path. However, the reel motors are selectively driven and a tachometer is associated with only the takeup reel shaft to provide a single pulse per revolution, which pulses are accumulated in a counter as a continuing count indicative of the radius of the tape wrapped around that reel. There is no derivation of the radius of tape wrap on the basis of the amount of tape advanced during rotation of one reel repeatedly during successive complete revolutions of the other reel. As noted in U.S. Pat. No. 3,984,868, inaccuracies in speed control resulting from variations in tape thickness and hub dimensions in the counter embodiment disclosed in U.S. Pat. No. 3,764,087 were eliminated during writing (but not reading) by making use of the spacing between the read and write gaps normally existing in a conventional read/write magnetic head to provide a measurement of actual tape speed during a writing operation. Tape speed measurements are used to produce correction pulses that are applied to the counter for reducing or increasing its count so that the resultant output of a linearity correcting circuit would provide a motor control signal more accurately corresponding to actual tape speed. Note that only one reel is driven at a time, and no means is disclosed or implied to provide tension control or acceleration control.